DEAR DR. DONOHUE: I have psoriatic arthritis. Can you tell me a little about this arthritis? What causes it, and is there a diet I should be on? Would it be better for me to live in a warm climate? ­- S.C.

For some – more than 7 percent – psoriasis is not just a skin disorder. It’s a combination joint and skin affliction. Peculiarly, the severity of joint disease doesn’t always parallel the severity of skin disease, and in a few – about 15 percent – joint involvement predates skin psoriasis.

It’s reasonable to suppose that the skin and joint manifestations have the same cause or causes. Those causes include genetics. Both manifestations run in families. The immune system is involved in both. And there has to be some environmental factor that activates the illness. The entire picture is anything but clear, but the picture becomes less fuzzy with each passing year.

Psoriatic arthritis behaves like most other kinds of arthritis, with joint swelling and joint pain. Small joints are particularly vulnerable to this form of arthritis – the knuckles nearest the fingernails and toenails, for example. Larger joints, however, don’t escape. The knees and elbows, as well as other such joints, can be involved with it.

I haven’t heard of a particular diet for psoriatic arthritis, nor have I heard that a warm climate eases it. Many arthritics, regardless of the type of arthritis, do feel better when it’s warm. Their joints are more moveable. Before planning to relocate, be sure to stay for a time in a warm climate to see what its effect is on you. The list of useful medicines for psoriatic arthritis grows. For mild cases, NSAIDS are often chosen. Slow-release indomethacin has the attraction of having to be taken only once a day.

Methotrexate, also valuable for skin psoriasis, has been quite successful in treating the joint complications of the illness. Newer drugs, which negate the inflammatory products that stir up arthritis, have recently come to the fore. Examples are Enbrel, Remicade and Humira.

DEAR DR. DONOHUE: What are the facts on sickle cell anemia? My relative’s infant child has it. I thought this was found here only in African-Americans. We are Caucasian. Will this baby reach adulthood? – T.O.

Sickle cell anemia is an inherited disease. In North America, African-Americans are the ethnic group having the greatest number of sickle cell patients. However, many other groups carry the sickle cell gene and can develop the illness: Greeks, Italians, Arabs, Turks, Iranians, Egyptians and Asiatic Indians.

To inherit the anemia, an infant must receive a sickle cell gene from the mother and the father. (Having only one gene confers the state known as sickle cell trait, something that usually produces no big troubles.)

Having two genes causes red blood cells to transform from their normal round shape to the shape of a sickle when blood oxygen levels drop, as they often do when the person becomes dehydrated.

Sickled red blood cells are sticky and form plugs that block blood flow through vessels. That leads to death of parts of bone and muscle. It also happens in the digestive tract, where sections can die. Kidneys are often involved in the sickling process. Ulcers might break out on the skin of the lower legs.

When there is widespread sickling, the episode is very painful and must be treated in the hospital with intravenous fluids for hydration. If the red-blood-cell count drops very low, then blood transfusions are required. Pain medicines are essential to ease the great discomfort that comes with these crises.

The drug hydroxyurea is prescribed when a sickle cell patient has frequent sickling crises. Your relative’s baby should make it to adulthood and beyond.

DEAR DR. DONOHUE: What is polydipsia? My 60-year-old brother is said to have it. He also has severe mental problems and lives in a group home. Will this new illness shorten his life? – R.W.

Polydipsia is drinking prodigious volumes of liquid, often water. It’s not exactly an illness. It’s a sign of an illness. Untreated diabetics have polydipsia because they lose so much fluid in the urine.

Diabetes insipidus, a completely unrelated kind of diabetes, also features polydipsia. That illness also causes a loss of urine in great quantities because the production of the hormone that regulates normal urine output has diminished.

Sometimes polydipsia is a psychological problem. If that’s the case, water intake has to be watched carefully.

If treated, no form of polydipsia should shorten life.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475.

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